The use of membranes for gas separation is becoming increasingly more common. In these systems, a mixture of gases, the feed gas, under relatively high pressure is passed across the surface of a membrane adapted to act as a selective barrier, permitting some components of the gas mixture to pass through more readily than others. Membranes used for gas separation processes wherein the separation mechanism is controlled principally by solubility and diffusivity, as opposed to free molecular diffusion, are classified as nonporous membranes. Nonporous membranes have a dense control layer which is crucial to membrane performance, and this layer can be adversely affected by moisture, chemical degradaton, or physical deformation.
Gas transfer through nonporous membranes is dependent upon the membrane surface area, the pressure differential across the membrane, the diffusion rate of the gaseous components, and the effective thickness of the membrane. Generally, the membrane layer through which the gases must diffuse should be as thin as possible to maximize gas diffusion rate. Membrane thinness, however, is limited by a need to have a membrane free from defects, such as pinholes, and the need to have a membrane which has sufficient physical integrity to withstand pressures up to about 4,000 pounds per square inch-gauge (psig) across the membrane. For example, asymmetric cellulose ester membranes can be produced which do have a very thin but dense (nonporous) layer and a supporting sublayer of larger pore size. The thin dense layer controls the mass transfer in the system, and the thicker sublayer provides structural integrity. Many types of membranes, including cellulose esters and polymeric membranes, such as silicate rubber, polyethylene and polycarbonate, may be employed in gas separation. The particular membrane used, however, depends upon the separation sought to be effected.
Triethylphosphate is a commonly used casting solution component in processes for making casted cellulosic polymer, especially cellulose acetate butyrate, membranes. Moreover, U.S. Pat. No. 3,607,329 to Manjikian states that it is essential to include from 15 to 25 weight percent triethylphosphate in solutions used to cast cellulose acetate butyrate membranes.
Commerical gas separation processes are generally continuous processes in which a feed gas stream is brought into contact at the feed side of a membrane. The pressure on the feed side of the system is maintained at a pressure sufficiently higher than the pressure on the permeate side of the membrane to provide a driving force for the diffusion of the most permeable components of the gaseous mixture through the membrane. The partial pressure of the more permeable gaseous components is also maintained at a higher level on the feed side of the membrane than on the permeate side by constantly removing both the permeate stream and the residue of the feed stream from contact with the membrane. While the permeate stream can represent the desired product, in most gas permeation processes the desired product is the residue stream, and the permeate stream consists of contaminants which are removed from the feed stream.
For example, CO.sub.2 and H.sub.2 S can be removed from a hydrocarbon mixture, such as natural gas, using a thin dried supported cellulose ester membrane, and a differential pressure across the membrane of about 100 psi. The partial pressures of CO.sub.2 and H.sub.2 S in the permeate stream are preferably kept at about 80 percent or less of the partial pressure of those same components in the feed stream by separately and continuously removing the depleted feed gas (residue) stream and the permeate stream from contact with the membrane. The residue stream can, of course, be fed to another gas separation membrane stage, and the permeate gas stream can likewise be fed to another separation stage to produce a product having a still higher concentration of the more permeable products. In fact, the use of multiple separation steps in series and/or in parallel offers considerable diversity in separation alternatives using membrane technology so long as sufficient pressures can be maintained in the system.
Feed stream pressures can vary from 10 to 4,000 psig, but are generally within the range of about 500 psig to about 3,000 psig. The differential pressure across the membrane can be as low as about 10 pounds per square inch (psi) or as high as about 2,100 psi depending on many factors, such as the particular membrane used, the flow rate of the inlet stream, and the availability of a compressor to compress the permeate stream, if such compression is desired. A differential pressure of at least 100 psi is preferred since lower differential pressure may require more modules, more time, and compression of intermediate product streams of modules arranged in series. Differential pressures of 1,200 psi or less are also generally preferred since the useful life of membranes is generally greater.
Spiral wound membrane arrangements are becoming more commonly used in commercial gas separation processes. An advantage of using a spiral wound technique is that this affords a large membrane contact area while permitting a rather small overall containment vessel. A standard way of supplying spiral wound membranes for commercial use is in the form of membrane units which comprise a section of permeate conduit around which the membrane is wound. These membrane units may then be used singly or joined together in series by interconnecting their permeate conduit sections. The usual way to use spiral wound membrane units is to contain them, either singly or multiply in modules. The modules can then in turn be used singly or can be conveniently interconnected in series or parallel arrangements to provide the desired treatment.
For many years cellulose ester membranes have been employed in liquid separation systems, such as reverse osmosis processes for the desalination of water, as well as in gas separation processes. Membraneous materials have been produced from various esters, including cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose cyanoethylate, cellulose methacrylate, and certain mixtures thereof. Cellulose acetate esters have become particularly favored for producing asymmetric nonporous membranes for gas separation. Such membranes comprise a dense nonporous layer which overlies a more porous layer; and can be cast on cloth from solution, then heat treated, and then dried, typically using a solvent exchange procedure intended to remove water. Free standing membranes can be prepared by casting on a non-adhesive material (e.g. silicone-coated paper), and then separating the membrane from that material after it has gelled, but prior to heat treatment and drying. While free standing membranes have generally exhibited satisfactory performance, they are generally considered too fragile for commercial applications. Drying is necessary for application to gas separation, but simple evaporation of water can cause shrinkage and a loss of the membrane's asymmetric character. The solvent exchange procedure thus serves to prevent the membrane from shrinkage and loss of performance.
Although cellulose acetate esters have shown considerable utility for various gas separation applications, they are not without disadvantages. They are relatively brittle in the dry state, and thus, their dependability over the long term may be affected by system disorders, particularly while adverse system pressure fluctuations are experienced. Moreover, even under normal operation, these membranes frequently exhibit a decline in permeation rates over time, particularly if there is a high moisture content.
In an effort to develop membranes with improved flexibility over cellulose acetate membranes, cellulose acetate butyrate membranes, both with and without heat treatment, have been studied for gas separation systems. Cellulose acetate butyrate membranes, like cellulose acetate membranes, had a history of use in liquid systems where they exhibited relatively favorable selectivity for a variety of inorganic and organic solutes. These membranes have been cast from a variety of formulations. However, heat treatment of cellulose acetate butyrate membranes above about 50.degree. C., as typically used for cellulose acetate membranes, was generally avoided because it could substantially reduce performance of the butyrate membrane. Reference is made to U.S. Pat. No. 3,607,329 as an example of such membranes. Cellulose acetate butyrate membranes have in the past generally been considered poor performers when compared to the cellulose acetate membranes.
In a report published in October 1976 Schell et al. published a report of attempts to use cellulose acetate butyrate membranes for gas separation. Schell, W. J. et al., Membrane Applications to Coal Conversion Processes, U.S. Dept. of Commerce National Technical Information Service, FE-2000-4. This report describes gas permeation rates and selectivity factors which were measured for a series of cellulose acetate butyrate membranes, Id at pp.86-89. Because membranes containing cellulose acetate butyrate were found to have poor performance characteristics, low permeation rates or low selectivity, these authors concluded that these membranes were not suitable at least for large scale gas separation processes and thus discontinued further work on cellulose acetate butyrate membranes.
The useful life of gas separation membranes, including in particular spiral wound membranes, has not been entirely predictable. Various factors are believed to affect the performance of membranes over time. These include the normal operating pressure differentials, the character of the gas being treated, the quality of the membrane itself and system disorders to which the membrane is subjected. A continuing challenge for those seeking to use gas separation membrane systems has been to improve the reliability of membrane systems, especially by prolonging the useful life of the membranes used therein.
One particular application of gas separation membranes is for separation of ammonia plant purge gases to provide a suitable hydrogen stream for reuse in ammonia synthesis. Many membranes, however, are adversely affected by ammonia in the purge gases. This necessitates pretreatment of the purge gases to remove ammonia.